Weird cruise control / mpg issue.

You understand the concept of exponential change, right?

An increase of headwind will increase the drag also if that is a factor between your two runs.

That is by design. State/Federal rules etc. Manufacturers err on the side of caution as the penalties for being on the wrong side are not going to be fun for them.

It has, but you're paying attention to the MPG in this car. In other cars, you weren't and you probably couldn't have cared less about what the MPG was either. (You probably did not do this side-by-side experiment either, but if you did you probably would not have thought much of 15 MPG vs 12 MPG. LOL.)

 

Parasitic drag Fd increases with the square of the speed v. 10 mph increase will increase drag by 100. Drag Coefficient Cd also increases if the undercovers are removed or the wheel covers are missing. Finally a bad wheel bearing or failed brake can add drag.

 

Bob Wilson posted an mpg-vs-speed chart here very long ago, for the 2010. Unfortunately, the original graphic vanished during one of the numerous host site migrations. It lives on copied into more recent other threads, but I don't know which. Gen2 charts for your 2008 were similar but a bit lower.

I'd have expected something closer to a 10 mpg loss, though Bob's chart did have some unusual variations between certain points.

Did breeze, or traffic conditions producing mild drafting, change? Did temperature change?

I understand it to be a low-order polynomial function, e.g. speed squared, not exponential.

Bob's chart, and similar ones on Wayne Gerdes' site, found no sweet spot in that speed range. MPG continually improves as speed slows, at least all the way down to Bob's minimum test speed of about 25 mph. Other threads employing additional tests and analyses, put the Prius best MPG at 'about 15 mph' from one group, and 'between 10 and 20 mph' from another group. All of these reveal that Prii exceed 100 MPG at such slow speeds.

Of course, due to highway legal minimums, traffic safety, and the value of time, nobody in the U.S. goes that slow on the highways. 100+ MPG isn't that valuable.

 

Prius also has insanely low other losses, so air drag is still a very large part of the whole picture. Perhaps even larger, compared to pre-hybrids.

 

At production the Gen2 had the lowest co of drag on production. I thought.

 

Wen I removed covers off wheels and plast trays underneath . Stay at around 47.1 still. On gen3 my seat under trays had 22 pounds each of rocks in . Removed so no more scooping rocks to tune of 88 pounds . Made no difference. No matter what I do wife gets 42 to 45 in gen3 w new engine running great w no issues or codes gas oil etc no.matter . I'm guessing it has to do w her driving. It's not worth looking into. Believe me..

 

I take your point, I'm all about being safe and driving safely as well. However, if a person wanted to do a 55 mph run over 10 miles, I'm sure there would be a suitable stretch of road where such a test could be performed safely.

You misconstrue what I meant, obviously what they proved is not disputed, but in my use of sweet spot, it is in the context of doing a reasonably fast speed and in the range of 50 to 70 MPH. I found a couple of threads from the same time period as Bob's post with data that more or less matches Bob's data. It bears out my experience, and you can see that it does pretty good in the 65-75 MPG but it does much better at 50-55 MPH and conversely much more horribly at the jump from 75 MPH to 80 MPH. But yeah, if your time is precious then you probably don't care too much. I will point out that faster speeds really only will make a tangible gain on trips that are 2 or 3 hours or more.

That is actually around the ballpark, although your 54 and 38 MPG are lower than the corresponding figures used as the basis of the chart below (61.5 and 50 MPG respectively). This is most likely the age of your battery. The figures in the graph are from 2005 when the car was quite new.

I have used gal (US) per 100 miles as this produces a linear graph as opposed to MPG which is not linear as the MPG gets higher and higher. For this reason, the higher the MPG gets the more it is exaggerated whereby a 3 MPG difference between 75 and 72 MPG is way less significant than a 3 MPG difference between 41 MPG and 38 MPG.

I have presented the data 3-ways; the two left ones are linear so you can work out differences and percentages across the whole range.

 

Overall, in my extended region, there are plenty of roads were 55 or 50 or 45 or 35 are suitable speeds. Though across my continent, not everyone has a full suite of these choices conveniently located near enough to them.

I find it misleading to declare a "sweet spot" that is constrained by an undisclosed window of acceptable speed specific to just your own circumstances. It is just in the eye of the beholder.

I drive in a number of places where reasonable or safe or even maximum legal speeds are below your expressed window, and don't want to see people speeding up there in order to get up into some mythical mpg "sweet spot," when they will get even better mpg by staying down to legal and safer speeds for those roads.

A better expression of the Prius's mpg "sweet spot" is "the very slowest speed you can otherwise reasonably accept, or 15 mph, whichever is faster". If your acceptable speed window is really 50 to 70 mph, then your "sweet spot" is 50, not 55.